A fiber laser light source characterized by its high oscillation efficiency, excellent beam quality, capability of employing air cooling, and simple structure has been attracting attention recently as a near-infrared laser light source that replaces a solid-state laser light source that has been conventionally used.
FIG. 7 shows a schematic view showing the configuration of a typical fiber laser light source. Laser light emitted from a pump LD 101 goes incident on a rare-earth-doped clad pump fiber 103, which is a laser medium, and the laser light oscillates as it is resonated in a laser resonator formed of fiber gratings 102 and 104, which are reflection mirrors.
A polarizer 105 is interposed in order to align polarization directions of the oscillated laser light to a single direction.
The fiber laser light source has a satisfactory beam quality and is also capable of regulating the oscillation wavelength spectrum to a line width of the reflection spectrum in the fiber grating 104 on the exit side. The fiber laser light source is therefore extremely suitable as a fundamental light source for higher harmonic generation (referred to as a wavelength conversion light source) using non-linear optical crystal.
A second-harmonic generation (SHG) module 108 in FIG. 7 is a mechanism that generates second harmonics. With the use of this mechanism, a second harmonic 107 at twice the fundamental frequency is emitted in the end.
In addition, with the conventional solid-state laser, the oscillation wavelength of the laser is regulated by laser crystal being used. On the contrary, with the fiber laser, the oscillation wavelength is also regulated by a pair of the fiber gratings 102 and 104. Hence, although the gain varies with the wavelength, the fiber laser is characterized by the capability of changing the oscillation wavelength arbitrarily.
Meanwhile, a laser display is attracting attention as an application using such higher harmonics of laser light as the light source (wavelength conversion light source) (Non-Patent Document 1).
Because the occurrence of unwanted infrared rays and UV rays can be suppressed in comparison with a white lamp that has been used, power consumption can be reduced. Moreover, using a laser makes it possible to collect light efficiently, which can in turn enhance light utilization efficiency.
In addition, because the laser emits monochromatic light and therefore has high color purity in comparison with a case where a light emitting diode is used, it is possible to enhance the color reproducibility of the display. In particular, by setting the wavelength of green light to 520 to 535 nm, it is possible to express deeper green.
FIG. 13 shows a color reproduction range for each wavelength of green light used in a case where the wavelength of blue light is 460 nm and the wavelength of red light is 635 nm on the chromaticity diagram. Wavelengths that can be generated in the case of using a solid-state laser are only two wavelengths: 532 nm when a Nd:YAG or Nd:YVO4 laser is used and 527 nm when a Nd:YLF laser is used. In particular, because YLF is fluoride crystal and is therefore difficult to manufacture, a fiber laser having a broad fluorescent spectrum (Non-Patent Document 2) and capable of choosing the oscillation wavelength without restriction has shown great promise.
As is described in Patent Document 1, because pumping light and oscillation light propagate on the same fiber in the fiber laser or the fiber amplifier, part of oscillated light becomes accidental return light and may possibly damage the pump light source. Such being the case, a method of avoidance to remove oscillation light using a lens system and a mirror has been discussed.
Regarding the configuration of a fiber amplifier that amplifies signal light (seed light) by inputting the signal light into a rare-earth-doped fiber together with pumping light, those disclosed in Patent Document 1 and Patent Document 2 are typical. It is also possible to amplify signal light generated in an oscillator using a fiber amplifier by combining the laser fiber and the fiber amplifier.
It is preferable for the green light source in the laser display to have a wavelength of 525 nm to 510 nm in terms of the color reproduction range. However, in a case where the wavelength conversion light source employing the fiber laser as the fundamental light source is used, the operation of the laser resonator (oscillation) becomes unstable because there is absorption of light at 1075 nm or shorter, which is the fundamental harmonic within the wavelength range specified above, in the rare-earth-doped fiber, which is a laser medium. This makes it impossible to extend the fiber length, which is an interaction length. This phenomenon becomes noticeable in a polarization maintaining fiber, such as a PANDA (Polarization-maintaining AND Absorption-reducing) fiber, used to obtain linearly polarized light that is essential in the case of the wavelength conversion light source.
Meanwhile, it is necessary to increase the pumping light in order to increase an output of laser light. However, depending on the wavelength of the pumping light, the pumping light that was not absorbed in the rare-earth-doped fiber, which is a laser medium, causes a problem that the fiber deteriorates. The mechanism of deterioration will be shown using FIG. 8.
FIG. 8 shows a fusion spliced portion 210 between a rare-earth-doped double clad polarization maintaining fiber and a typical single mode polarization maintaining fiber. The double clad polarization maintaining fiber is of a structure that allows light to propagate through an inner clad 203 while residual pumping light 208 is confined within an outer clad 202.
On the other hand, after it is connected to the single mode polarization maintaining fiber, air serves as the clad and the residual pumping light 208 is confined in a portion where a coating 207 is absent on the single mode polarization maintaining fiber. However, the pumping light leaks in a portion where the coating 207 is present. With this energy, the single mode polarization maintaining fiber partially generates heat (for example, heat generation portions 209) and thus deteriorates.
In this instance, assume that the pumping light is at power of 10 W, then an absorption amount of the double clad polarization maintaining fiber doped with Yb as a rare earth is 0.6 dB/m. Hence, 7.5 W of the pumping light is absorbed over the fiber length of 10 m. Accordingly, 2.5 W of light at 915 nm is irradiated as the residual pumping light and propagates through the clad of the single mode polarization maintaining fiber.
With the conventional configuration shown in FIG. 7, in a case where pumping is performed with 15 W of pumping light (915 nm) so that an output of oscillated light at 1064 nm is 6.8 W, the fusion spliced portion 110 and the primary coat (coating) on the single mode polarization maintaining fiber 112 overheat after 20 minutes since the continuous operation started and the fiber deteriorates.
FIG. 9 is a plot diagram showing the relation between the fiber length of an Yb-doped double clad fiber, which is a laser medium, and the residual pumping light with the power of the pumping light as a parameter. It is known from the examinations in the past that the fiber deteriorates when the residual pumping light exceeds 3.5 to 4 W. It is understood from FIG. 9 that it is necessary to make the pumping light smaller in a case where the fiber length has to be shortened due to an intrinsic loss of the fiber. More specifically, a possible output is naturally limited at the wavelength of 1050 nm or 1030 nm at which the loss in the fiber is significant.
In a case where light is oscillated at 1070 nm or longer, which is the wavelength at which oscillated light is not absorbed into the fiber, overheating of the fiber can be prevented by extending the rare-earth-doped double clad polarization maintaining fiber. However, at the wavelength, such as 1060 nm and 1050 nm, at which green can be produced through wavelength conversion, it is known that extending the length of the rare-earth-doped double clad polarization maintaining fiber makes a considerable loss resulting from fiber absorption, which raises problems that the oscillation becomes unstable and the light fails to oscillate at a desired wavelength. The intensity of pumping light to avoid overheating of the fiber is therefore determined naturally. Limitation is thus imposed on the maximum output.
FIG. 10 shows the absorption spectrum of the rare-earth-doped double clad fiber doped with about 1000 ppm of Yb as a rare earth, and a conventional example of the method for eliminating such a phenomenon will now be described.
A laser diode (LD) with the wavelength in the neighborhood of 915 nm or a laser diode with the wavelength in the neighborhood of 976 nm can be used as the pumping light. In this instance, an absorption amount of the fiber for 915-nm light is about 0.6 dB/m whereas an amount of the fiber for 976-nm light increases by three times to about 1.8 dB/m. Hence, using 976-nm light is thought to eliminate deterioration of the fiber.
However, the shape of the absorption peak is steep in the neighborhood of 976 nm and broad in the neighborhood of 915 nm. Accordingly, it is more stable to use a 915-nm band (900 to 950 nm) against a wavelength fluctuation of the pumping light caused by a temperature change of the pumping light LD or the like, and the cooling mechanism of the LD can be simplified. The device costs and power consumption can be consequently reduced. As has been described, it has been difficult to achieve the temperature stability of the fiber laser device and to obtain linearly polarized light at 1070 nm or shorter having power of 10 W or higher using the fiber laser at the same time.
Meanwhile, as a totally another technique, there is a technique known as a fiber amplifier that amplifies an output of signal light by inputting the signal light and pumping light into the rare-earth-doped double clad polarization maintaining fiber. FIG. 11 shows the configuration of this conventional technique.
Semiconductor laser light or a fiber laser of the single mode can be used as the signal light. FIG. 11 shows a case where a fiber laser is used as the signal light and a description will be given on the basis of this drawing.
As is shown in FIG. 11, a laser light source of this conventional example includes a fiber laser portion and a fiber amplifier portion.
The fiber laser portion is formed of a fiber laser pump LD (laser diode) 101 and a laser resonator. The laser resonator is formed of an Yb-doped clad pump fiber 103, a fiber grating 102, a fiber grating 104, and a polarizer 105. The Yb-doped clad pump fiber 103 and the fiber grating 102 are made of a double clad fiber and the fiber grating 104 and the polarizer 105 are made of a single mode fiber.
The fiber amplifier portion is formed of a fiber amplifier pump LD 501, an Yb-doped double clad fiber 503, and a pump combiner 502. The pump combiner 502 is made by integrating a single mode fiber 504 to input signal light with a multi-mode fiber 505 to input pumping light. The fiber amplifier portion is made of a double clad fiber.
Signal light generated in the fiber laser portion is amplified in the fiber amplifier portion in the latter stage.
In the configuration of this conventional example, however, not only is it necessary to prepare the fiber laser pump laser diode 101 and the fiber amplifier pump laser diode 501 separately, but it is also necessary to prepare the pump combiner 502 formed by integrating the single mode fiber 504 with the multi-mode fiber 505. The material costs are increased because of these two factors.
Moreover, because points at which the fiber is fusion spliced are increased, there is a drawback that the device becomes less reliable.
In addition, depending on the oscillation wavelength, signal light that can be generated in the fiber laser portion fails to achieve satisfactory intensity. This raises a need to adopt a multi-stage configuration by connecting more than one fiber amplifier portion in series, and thereby poses a problem that the material costs are increased.
Further, the optimal fiber length of the Yb-doped fiber varies with the wavelength. The fiber length of the Yb-doped fiber is determined so that light-to-light conversion efficiency, which is found by dividing the output of generated light by input power of the pumping light inputted, reaches the maximum.
FIG. 12 shows the relation between the fiber length and the light-to-light conversion in a case where a polarization maintaining fiber is used at the generation wavelength of 1064 nm. In this case, the optimal fiber length is 17 to 18 m. The optimal value actually varies slightly due to a fusion loss or the like. However, it can take the maximal value of the conversion efficiency by varying the fiber length. The fiber length at which the light-to-light conversion takes the maximal value tends to become shorter as the oscillation wavelength becomes shorter. Hence, the pumping light has to be decreased as the oscillation wavelength becomes shorter, which makes it difficult to achieve a high output. This tendency is noticeable with the polarization maintaining fiber and the fiber length is about half the Yb-doped fiber length used in a single mode fiber for normal random polarization.
As has been described, the fiber laser and the fiber amplifier of the conventional configuration in FIG. 11 is available for laboratory use but is not readily applied to industrial and commercial use.    Patent Document 1: Japanese Patent No. 3012034    Patent Document 2: JP-A-2-43782    Non-Patent Document 1: Japanese Journal of Applied Physics, Vol. 43, No. 8B, 2004, pp.5904-5906    Non-Patent Document 2: Rare-earth-doped Fiber lasers and amplifiers, (Marcel Dekker, Inc. 2001), p.145, FIG. 10